Transforming growth factors are recognized as an important class of factors because they confer a malignant phenotype on non-neoplastic cells. At present the functions of these factors are poorly understood. This project examines the roles of transforming growth factors during early mammalian development. Using early mouse embryos and embryonal carcinoma cells, it is shown that early embryonic cells release factors with transforming growth factor activity. However, studies with embryonal carcinoma cells demonstrate that these cells release an entirely different category of transforming growth factors. Unlike the transforming growth factors purified and characterized previously, the factors released by embryonal carcinoma cells do not bind to receptors for epidermal growth factor nor do they act synergistically with epidermal growth factor. Attempts thus far to identify the factors released by embryonal carcinoma cells suggest that several different factors are released, including one that exhibits the properties of platelet-derived growth factor. It is not yet known whether this particular factor is responsible for any of the transforming growth factor activity released by embryonal carcinoma cells, but highly purified platelet-derived growth factor itself does stimulate the growth of endoderm derived from embryonal carcinoma cells and early mouse embryos.